The present invention is directed toward a flow control circuit for regulating orifice gas flow to a plasma arc torch in plasma arc welding apparatus and more particularly to such a flow control circuit including means for selectively increasing, decreasing and maintaining at a steady state the flow of orifice gas to the torch.
In conventional plasma arc welding apparatus of the type contemplated by the present invention, the torch electrode and welding stock are electrically coupled to form a cathode and anode respectively. A voltage is applied between the torch electrode and welding stock so that an orifice stream of gas such as argon or helium is ionized in order to produce a plasma jet stream which is directed toward the welding stock. Increasing orifice gas flow causes both the heat input and arc force on the weld puddle to also increase. Both of these factors improve weld penetration into the welding stock. An inert shielding gas commonly surrounds the plasma jet stream in order to prevent contamination from the atmosphere.
Plasma arc welding carried out with apparatus of the type referred to above is capable of rapidly completing a weld because of the high temperature produced by the plasma stream. Plasma arc welding is also particularly adaptable for precision, high-quality and deep penetration welding because of the concentration of large amounts of energy in the plasma stream from the plasma arc torch and high arc force.
During welding operations carried out with such equipment, it is necessary to regulate numerous parameters in order to effectively achieve high quality welds and reproducible results. Possibly the most important control parameters are the electrical potential applied to the torch and the flow rate at which the orifice gas is introduced into the torch for conversion to plasma. The present invention is not concerned with adjustment of the electrical potential of the torch. However, insofar as the present invention relates to the regulation of the flow rate of the orifice gas, the flow rate of orifice gas may be interrelated with adjustment of the electrical potential of the torch.
Plasma arc welding apparatus is commonly set up for operation in an automatic welding cycle which selectively increases, decreases, and/or maintains at a steady state the flow of orifice gas toward the torch according to a predetermined plan. A particular example of such a cycle may be seen in the welding of pipe or tubing for example where an initial portion of each circumferential weld is overlapped by the ending portion of the same weld. Within such a cycle, it is important to accurately upslope or increase the flow of the orifice gas in order to initiate the weld. Thereafter, the flow rate of the orifice gas may be maintained at a steady rate until the end of the welding cycle is approached. At that time, the flow of the orifice gas must be accurately downsloped or decreased in order to properly terminate the weld. In some applications, such as during the welding of tubing and pipe, the torch encounters portions of the welding stock which were substantially preheated during an earlier part of the welding cycle. At such times, it is often necessary to taper or very gradually diminish the rate of orifice gas flow in order to maintain uniform weld quality. The tapered portion of the weld is terminated prior to or at the time that downsloping of the orifice gas flow commences.
Within the prior art, the rate of plasma gas flow was adjusted by relatively complex flow circuits including a series of gas reservoirs which could be filled and/or emptied in order to accomplish upsloping and downsloping of the orifice gas flow at various preselected rates. Such a technique is illustrated and described in U.S. Pat. No. 3,471,674, issued Oct. 7, 1969, and U.S. Pat. No. 3,602,683, issued Aug. 31, 1971.
Flow control circuits of the type referred to above are susceptible to gas impurities collecting within the reservoirs and pressure surges within the flow circuit. Also, upsloping and downsloping of the orifice gas flow rate may not be linear in accordance with requirements of the plasma arc welding process.
Accordingly, there has been found to remain a need for an accurate control circuit capable of regulating the flow of orifice gas to the plasma arc torch of plasma arc welding apparatus.